The anti-CD20 antibody, rituximab, is effective as a single agent for the treatment of human B-cell lymphomas. Despite a 50% response rate in intermediate grade lymphomas, the complete response rate is low, and no patients are cured of their disease. The mechanism of rituximab's activity against lymphoma remains unclear; almost all of the existing data is from in vitro studies using lymphoma cell lines, and suggests that rituximab functions through antibody- dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), direct induction of apoptosis, or a combination of these mechanisms. We have utilized a murine model of a spontaneously arising human malignant lymphoproliferation to study the mechanism of rituximab's cytotoxic activity in vivo. In this model, severe combined immune deficient (SCID) mice are engrafted with human peripheral blood leukocytes (hu-PBLs) via intraperitoneal injection (the hu-PBL-SCID model). If PBL from selected normal Epstein-Barr virus (EBV)-seropositive donors are injected, all mice develop lethal EBV-derived lymphoproliferative disease (EBV-LPD) within 8-12 weeks. This EBV-LPD is CD20+, is of human origin, and arises in the midst of engrafted normal human immune effector cells including CD8+ T cells and NK cells. In this model, rituximab is capable of eradicating established EBV-LPD, but only in mice pretreated with daily low-dose interleukin-2 (IL-2). The mechanism by which IL-2 promotes the cytotoxic function of rituximab is unknown, and is the focus of this proposal. A leading hypothesis is that IL-2 exerts its effect through the modulation of activating and inhibitory Fc-gamma receptors (Fc-gamma- R), either on the tumor itself, or on required immune effector cells, thus leading to more effective ADCC. In the first aim we will determine the immune effector subset(s) necessary for the therapeutic effect of rituximab and daily low-dose IL-2. In the second aim, we will characterize the effect of IL-2 on the differential expression of Fc-gamma-RI, Fc-gamma-RII, and Fc-gamma-RIII on human and murine immune effector subsets, and determine the functional relevance of murine Fc-gamma-R molecules for the protective effect of rituximab and IL-2 therapy. For the third aim we will characterize the effect of daily low-dose IL-2 therapy on EBV-LPD tumors and their susceptibility to ADCC, apoptosis, and cell-mediated cytotoxicity. Insight gained from these studies should be applicable to the treatment of a variety of human malignancies for which antibody therapy currently exists, and may lead to further optimization of such therapies in the future.